linux: serial: Use proper types and fix a copy/paste error

While at updating the prototypes in the lab book, fix the reg_read() definition
which should return something anyway.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

labs/kernel-serial-iomem/kernel-serial-iomem.tex

@@ -153,7 +153,7 @@ \subsection{Accessing device registers}
 
 Your prototype should look like:
 \begin{verbatim}
-static void reg_read(struct serial_dev *serial, int reg);
+static u32 reg_read(struct serial_dev *serial, unsigned int reg);
 \end{verbatim}
 
 In this function, read from a 32 bits register at the base virtual
@@ -170,7 +170,7 @@ \subsection{Accessing device registers}
 the \kfunc{writel} convention of passing the value first, then the
 offset. Your prototype should look like:
 \begin{verbatim}
-static void reg_write(struct serial_dev *serial, int val, int reg);
+static void reg_write(struct serial_dev *serial, u32 val, unsigned int reg);
 \end{verbatim}
 
 In the next sections, we will tell you what register offsets to use